Scientists studying fossils from a 'mass grave' some 360 million years ago have provided new insight into a group of strange fish-like creatures.

Known as Acanthostega, they are believed to be a key evolutionary link in the move of backboned animals from the seas onto the land.

New analysis suggests that all of the fossils belong to young animals which died while living exclusively in the water in a region now belonging to Greenland.

The researchers say their surprise findings provide new insight into the lives of these mysterious animals, which may have waited until their adult years to leave the water.

Fossils of these early tetrapods, called Acanthostega, come from a group believed to have died in a river bed more than 360 million years ago

TETRAPODS AND THE TRANSITION TO LAND

Tetrapods were believed to have made the transition to land more than 300 million years ago, giving rise to all modern vertebrates on land.

Fossils from one of these creatures, the fish-like Acanthostega from 365 million years ago, have been analysed by researchers.

By using high energy X-rays they were able to look at microscopic growth structures in their front legs.

The findings suggest that all of the specimens were juveniles and spent their lives in the water.

Their forelimbs were mainly cartilage and would have been unable to bear their weight out of the water.

But the team notes that there is evidence of ossification – the laying down of hard minerals which make up bone – suggesting they could potentially have spent the first stage of their life in the water, before moving onto land after they mature.

Scientists say this remains the big unanswered question, with no adult fossils available, they are unable to know for sure.

The point at which the first tetrapods ventured onto land is seen as an evolutionary turning point, eventually giving rise to all modern backboned creatures on the land - including mammals, reptiles and birds.

But such a shift would have required some physiological adjustment, with animals having to move their own body weight without the helpful buoyancy of water.

By blasting fossilised limbs of the creatures with high energy X-rays at the European Synchrotron Radiation Facility (ESRF) in France, researchers have been able to see tiny developmental markers, revealing that at least in the first part of their lives, the animals never left the water.

A team led by researchers at Uppsala University in Sweden and the University of Cambridge were able to build up this picture of the animal's life cycle by analysing growth rings in their front legs.

‘Like a growing tree, a limb bone is marked by seasonal rhythms and lays down annual growth rings,’ explained Dr Sophie Sanchez, a researcher at Uppsala University and the ESRF, who lead the study.

‘These growth rings, which can be seen in both fossil and living tetrapods, are informative about the development and age of the individual.’

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Microscopic structures in the legs of ancient fish-like creatures, believed to have been a crucial link in the transition to land, show the creature were juveniles which spent their lives in the water

Their analysis revealed that all of the Acanthostega individuals which died were juveniles, aged around six.

But the growth rings of their upper front limbs suggest the legs were still largely made of cartilage when the animals died.

'In contrast to bone, cartilage is a non-mineralised tissue, elastic and far too weak to allow the forelegs to sustain the weight of the animal’s body out of the water,' explained Dr Sanchez.

Researchers say that the animals are not the only class of tetrapods whose bones apparently formed late. They suggest that the onset of ossification - the laying down of hard bone-forming minerals - is delayed in giant salamanders as well (pictured)

It is thought that those early tetrapods which made the move to land would likely have a similar life cycle to amphibians - where the eggs are laid in water and juveniles mature before moving onto the land.

Analysis of the juvenile Acanthostega fossils may rule them out as the first tetrapods to head for land, but the latter half of their lives remains a mystery.

One tantalising nugget remains in that the animals are not the only class of tetrapods whose bones apparently form late in their life cycle.

The researchers suggest that the onset of ossification - the laying down of hard bone-forming minerals - is also delayed in giant salamanders, which spend parts of their adult life on land.

With no adult Acanthostega fossils on record, the findings paint a picture of an aquatic animal which spent the first stage of its life in the water, with bone growth in the legs coming later in life, closer to sexual maturity.

Whether or not the bones of juvenile Acanthostega would have toughened up enough by the time it reached maturity remains a mystery.

Dr Sanchez told MailOnline: 'We don’t know if the adults were able to crawl onto land as we have no adult fossil specimen at our disposal. For sure, the juveniles were only aquatic.'